2dw.
Name;
Date
Section
Points. 10/10
Test 1. Topic 1. Steering control system.
Type or write your answer with a clean paper, screen shot the answer
and send to my messenger today.
1. The ______ in charge of the navigational watch shall make
regular checks to ensure that: The person steering the ship or the
automatic pilot is steering the correct course;
2. When the vessel is at sea normally the auto pilot is used. In
that case the set course of the vessel is compared with the real
course of the vessel as indicated by a _____repeater.
3. while on duty, regularly check the operation of the radio
equipment and its sources of energy and report to the_____ any
observed failure of this equipment
4. Within______ hours before departure, the ship's steering gear
shall be checked and tested by the ship's crew and entered in the
logbook.
5. When _____ course it is standard practice to consider three
phases of the manoeuver: the start of the turn, the period of
steady turn, the end of the turn.
6. ______steering and input commands are under the control of
the helmsman.
7. ______ can be performed from three different automatic
steering modes.
8. _______The primary mode, used for automatic heading keeping
with data input from a gyrocompass and the helmsman’s Order
setting.
9. _______An optional steering mode performing automatic
heading keeping using inputs from an external management
system to steer the vessel to pre-determined waypoints.
10. ______ An optional steering mode using inputs from an
external navigator, which is corrected for cross-track error by the
autopilot to steer the ship to a waypoint over a designated
ground track.
2dw
Topic 2 Echo Sounder.
Deck watch keeping
Marine echo sounding equipment
Electronic Navigation
Year 2021 – 2022
Deck Officers
Lead - Depthmeter - Echsounder
Reasons for knowing the local waterdepths are:
- Relation between draught – surrounding water
- Preventing damage by sailing onto a shallows
- Navigating waters with variable bottom conditions
- Depth at load- and discharge places
- Arrival and mooring at slooping shores/banks
Types of depthmeters
1. Handlead (sea- and inland navigation)
2. Thomsonlead (sea navigation)
3. Measuring rod (inland navigation)
4. Echosounder (sea- and inland navigation)
Handlead
The handlead consists of a line and a weight of lead connected to it.
The line can be marked at different intervals.
2m red cloth
4m white cloth
6m blue cloth
8m yellow cloth
10m leather with hole
20m leather with 2 holes
Thomsonlead device
In this device the hydrostatic pressure causes a
discoloration in a glass tube, filled with a
Measuring rod
The measuring rod was (is) a long wooden
stick
The bottom part (submersion empty ship)was usually painted black,
after that blocks of about 10 or 20 cm were alternately painted in
two colors, while at every metre a distinct bright color was used.
At the top of the measuring rod a Turkish knot was laid, held in
position with a small wedge.
The measuring rod was stuck overboard in the sailing direction and
one tried to find bottom. At the moment the Turkish knot emerged
this was called aft. The skipper could than reduce speed a bit or
change course. This device was also very useable when aground.
The echosounder transmits a bundle of
ultrasonic soundwaves to the bottom. The
propagation speed of these waves is known.
The time elapsed between the transmission
and the reception of the , by the bottom,
reflected soundwaves, is measured and is a
indication of the depth.
Ultrasonic sound
Advantages:
- Very little shipsinterference. Every ship produce noise. This
noise is primarely in the audible range
- It turned out that ultrasonic sound was better to bundle. With
the use of ultrasonic sound the dimensions of the transducers can be
much smaller than compared with audible sound.
Disadvantage:
- A disadvantage of ultrsonic sound, in comparison with audible
sound, is that the range of audible sound is much larger, because
audible sound is less absorbed by water.
Loss of signal is caused by reflection and
absorption
Refraction (breaking) and diffraction (bending)
The transmitted bundle of waves
Propagation speed
The propagation speed (through water) is approximately 1500m/sec
This depends on:
- salinity
- temperature
- pressure
-other (chemical) impurities
Principle of transmission and reception
Separated transducers
Blocking of the reception during
transmission
Amplifier Blocked
Echo sounding Sequence
Transmitter sends electrical power to transducer
Transducer converts power to mechanical energy and sends
sound wave into water
Sound wave propagates through water, reflecting off of
bottom surface
Returning signal compresses transducer which converts
mechanical energy into electrical energy which is measured
by the receiver.
Signal is displayed and recorded
Sound Speed
In order to determine depth time, need speed of sound in
water
Sound speed varies with temperature, pressure and depth
Refraction not an issue with vertical incidence echo
sounding
Harmonic Sound Speed
Harmonic Mean sound speed is the weighted average sound speed
down to the depth of interest.
Transducer Materials
Magnetostrictive
◦ property of ferromagnetic materials to undergo a change of
their physical dimensions when subjected to a magnetic
field
◦ allows for the conversion of mechanical energy into
magnetic energy (and the reverse)
Piezoelectric
◦ the ability of certain crystals to produce a voltage when
subjected to mechanical stress
◦ allows for the conversion of mechanical energy into
electrical energy (and the reverse)
◦
Piezoelectricity
In a piezoelectric crystal, the positive and
negative electrical charges are separated, but
symmetrically distributed, so that the crystal
overall is electrically neutral.
When a stress is applied, the symmetry is
destroyed, and the charge asymmetry generates
a voltage
crystals of tourmaline, quartz, topaz, cane sugar,
and Rochelle salt (sodium potassium tartrate
tetrahydrate) ceramic polymers etc..
Piezoelectric Transducer
Transducer Directivity
Point Source (no directivity)
Piston source (directional)
Beam Patterns
Image
courtes
yJ.E.
Hughes
Clarke,
UNB,
OMG
Beam patterns show the relative amplitude
of the acoustic pressure (generated or
received) as a function of direction relative
to the transducer. For reciprocal transducers
the transmit and receive beam patterns are
basically the same. Beam patterns are
three-dimensional
Beam Width
Measured at -3dB
◦ Half power
◦ Effect on echo sounding
◦ Coverage increases with depth
◦ Can distort bottom features
Hyperbolic echoes
Bottom Slope effect
Beam Width
Errors due to beam width
Bottom Detection
First Arrival
◦ Echo rise time
◦ Threshold level
Transducer Frequency
Low frequencies (≈ 2 – 50 kHz)
◦ Lower attenuation
◦ Requires larger transducer (vessel)
◦ Lower resolution
High frequencies (≈ 50 – 500 kHz)
◦ Higher attenuation
◦ Smaller transducer (launch)
◦ Higher resolution
Single Beam Calibration
Draft
Wire with length markings
Acoustic Range to
Cone
Cone
The strenght of the echo depends on:
1. Waterdepth
2. The bottom condition (hard, soft, silt etc.)
3. The angle the bundle hits the bottom with
4. With engine astern or using the bowthruster, airbubbles are
formed in the water. As generally known the speed of sound through
air is much lower than through water. Resulting in incorrect
information.
Most echosounders can be adjusted so that:
1. The readout is the total depth, hereby one needs to adjust for
own draught
2. Readout is depth under the keel between transducers and
bottom.
3. An alarmdepth. This consists of own draught +
safetymargin.
As soon as this depth is reached an audible (acoustic) sound
will start on the echosounder.
For example draught is 2,80 m, safety margin is 0,60 m.
Alarm set for 3,40 m.
At 3,40 m the alarm will start.
False echo’s can be caused by:
- school of fish
- airbubbles under the ship
- wake of other ships
- outlets of sewers and factories
Components of the echosounder.
Advantages of multi beam echo sounders
The benefits of multibeam echo sounders are that they map the
seafloor by a fan of narrow acoustic beams, thus providing 100%
coverage of the bottom. The resulting seabed maps are more
detailed than those obtained using single-beam mapping.
Used for surveying seabed
The most common system for measuring water
depth, and preventing collisions with unseen
underwater rocks, reefs, etc., is the echo
sounder.
These sonar systems use a transducer that is
usually mounted on the bottom of a ship.
Sound pulses are sent from the transducer
straight down into the water.
The time the sound takes to travel to the bottom and
back is used to calculate the distance to the seafloor.
Water depth is estimated by using the speed of sound
through the water (approximately 1,500 meters per
second) and a simple calculation: Distance = speed x
time/2
The product is divided by two because the measured
time is the round-trip time (from the transducer to the
seafloor and back to the transducer).
The faster the sound pulses return to the transducer
from the ocean floor, the shallower the water depth is
and the higher the elevation of the sea floor. The sound
pulses are sent out regularly as the ship moves along
the surface, which produces a line showing the depth of
the ocean beneath the ship.
Echo sounder
n These sonar systems use a transducer that is usually
mounted on the bottom of a ship
n Sound pulses are sent from the transducer straight
down into the water
n The sound reflects off the seafloor and returns to
the transducer
n The time the sound takes to travel to the bottom
and back is used to calculate the distance to the
seafloor.
n Water depth is estimated by using the speed of
sound through the water (approximately 1,500
meters per second) and a simple calculation:
Distance = speed x time/2
n The echo sounders measures the Under Keel
Clearance (UKC)
End of topic 2 and short test next meeting.
